copper would be better filler ...;-)
jk
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]]On Behalf Of
>[log in to unmask]
>Sent: Monday, September 30, 2002 7:39 PM
>To: [log in to unmask]
>Subject: Re: [TN] PCB design with mask over vias.
>
>
>There is a theory in my Company that solder-filling holes, especially in
>the likes of FR4 with a relatively high LCTE in the Z axis, negates the
>ductility of the Cu plating inside the holes. Without the ductiity of the
>plating, the risk of shearing the barrel at the interconnections with inner
>layers is greatly increased with a corresponding decrease in board
>reliability. I'm talking about high-rel boards here, and I subscribe to
>this theory. Any thoughts, anyone?
>
>Peter
>
>
>
>Dave Chapman <[log in to unmask]> 30/09/2002 06:02 PM
>Sent by: TechNet <[log in to unmask]>
>
>Please respond to "TechNet E-Mail Forum."; Please respond to Dave Chapman
>
> To: [log in to unmask]
> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
> Subject: Re: [TN] PCB design with mask over vias.
>
>
>
>
>
>
>
>
>One thing we just completed was a pilot where we put apertures in our
>topside SMT stencil and pasted over all vias to prevent Conformal Coat
>leaking through the board. We opened the apertures +10% and got good solder
>fill this also prevented corrosion or having trapped mask inside filled
>vias
>possible "volcano" during reflow.
>Dave Chapman
>Manufacturing Engineer
>Circuit Service Inc.
>
>
>-----Original Message-----
>From: Willie Duersch [mailto:[log in to unmask]]
>Sent: Friday, September 27, 2002 2:01 PM
>To: [log in to unmask]
>Subject: [TN] PCB design with mask over vias.
>
>
>Hello Peers:
>We are a CM who recently fabricated a PCBA which had the vias covered by
>solder mask film design on the PCB. This is not the first time we have seen
>this done in PCBA. However, in this example, the PCBA ended up inside a
>refrigerator. This customer adds few notes with the PCB and nothing was
>specified associated with the fab on plugging or capping of the vias. The
>PCBA did not require wave solder. Aqueous clean paste was used and PCBA
>washed after SMT.
>
>The PCBA was supplied to the customer who then selectively applied
>Conformal coating over the top side components on the board with a brush,
>but did not coat other through holes or vias on the component side or any
>area of the back side.
>
>Over a period of time, The vias on the back side showed evidence of
>corrosion and subsequent cross section analysis showed copper corrosion
>from the pad part way into the barrel even under soldermask. The cross
>section also showed that the LPI solder mask did not plug and did have pin
>holes in the mask with bare copper in the air pockets that occur during LPI
>screen deposition.
>1. We feel the customer is responsible to specify the PCB and PCBA
>performance criteria. Comments?
>2. Is there an IPC document that covers solder mask covered vias and
>guidelines associated with the practice? If not, why not?
>3. Is it the PCB shop or the CM who should question, and if not
>specified...should either question the practice?
>4. What are the issues related to designing with solder mask film design
>covered vias? In this case, the vias were .010-.014 diameter and space
>constraints were not an issue. There were other holes on the PCB that were
>larger diameter and were left open to coat with solder from the HASL
>operation...and not coated with conformal coat.
>5. What would you tell a customer to correct the issue?
>
>There are some obvious issues and answers and I have had 12 years of
>experience with PCB's. However, I really would like to have some comments
>from my peers.
>Best regards, Willie
>
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